What causes HD...?

Huntington's Disease is caused by a mutation in the HD gene, on chromosome 4. This gene is what provides information to make a protein, huntingtin. What huntingtin does is unknown to scientists at this time, they do know some things about it including it plays a very important role in nerve cells. What happens is when a DNA segment, a CAG repeat, become much larger than what it is supposed to be within the HD gene. It is supposed to be repeated 10 to 35 times, but people with CAG segment it is repeated 36 to more than 120 times. This then leads to producing a huntingtin protein that has a long stretch of the amino acid glutamine.